The propeller of a vessel has a shaft extended from the engine at one end and penetrates the body of the vessel at the other end, protruding out of the vessel body. From the difference in pressure between the inside of the vessel and the outside water which the vessel is submerged within, and from the forward thrusting force generated from the rotation of the propeller, a mechanism preventing water incursion through the gap in between the propeller shaft and the vessel hull at the propeller shaft-vessel body penetration point is required.
The presently claimed invention provides an apparatus that is a propeller shaft seal for completely preventing water incursion through the gap existing around the propeller shaft at the propeller shaft-vessel body penetration.
The traditional method of preventing water incursion through the gap between the hull and the propeller shaft of a vessel is described as follows. First, a stern tube is fixed in the hull of the vessel. Then, a pipe called stern tube bearing is installed inside the stern tube. The propeller shaft is inserted into the stern tube bearing prior to use. The propeller shaft and the stern tube bearing have lengths of more than three times the diameter of the propeller shaft. The sealing function is achieved by extending the route of incurred water flowing into the gap between the stern tube and the propeller shaft. The resistive force against the incurred water flow reinforces the sealing function. Lubricant is forcefully injected into the gap, mixing with the flow of the incurred water flowing through the pipe for providing cooling and lubricating functions. Lip seals are inserted into both ends of the stern tube bearing in layers to prevent the lubricant and the incurred water overflowing into inside of the vessel. Lastly, the mixture of the incurred water and the lubricant is collected in a container and the lubricant is recycled after separating the lubricant from the incurred water using an oil-water separator.
The abovementioned traditional method is used because there has not been better sealing device that can be used for sealing propeller shafts in vessels. Moreover, such traditional propeller shaft sealing method is inadequate for use in deepwater diving vessels. First, the rubber-made lip seals are inappropriate for vessels sailing in cold waters due to the effect of rubber embrittlement that causes fractures. Thus, the mechanical strength and sealing performance of the lip seals decrease under extreme low and high temperatures. Second, the rubber-made lip seals are also inappropriate for deepwater dives due extrusion at high pressure that causes fractures. Furthermore, as stern tube bearings are not of a rolling type but of a sliding type, they feature high friction loss and short lifecycles, along with a host of other shortcomings, such as high maintenance cost, large lubricant consumption, etc.
Underwater pressure increases by one bar for every ten-meter increment in water depth in either ocean or fresh water bodies. For military submarines and industrial submarines, a considerable amount of researches has been conducted addressing issues on rotational sealing of propeller shafts. These researches are in line with the technological development in increasing the strength of vessel hulls with the goal of enabling deeper and deeper submarine dives.
One of the issues on rotational sealing of propeller shafts is that the synthetic resin lip seals lose their sealing functions at temperatures below −30° C., such as those within the Arctic Circle, due to loss of elasticity. Overtime under the exposure of extreme low temperature, the synthetic resin lip seals break into small pieces and fall apart from the propeller shafts. This issue has become more profound recently as a significant volume of ice has been lost, making new shipping routes through the Arctic Circle possible.
Another issue is that many diverse types of minerals, which can be found in coastal areas and continental shelves, are spread across the seafloors at different depths ranging from 200 to 2000 meters below sea level. Some of these minerals are exposed on the seafloor surfaces as in the case of manganese nodules. Some others can be found by digging lightly into the seafloor as in the case of methane-hydrate, which is also dubbed “burning ice”. For these reasons, the perfect sealing of propeller shafts has become the utmost important technological pursuit in the marine industry.
There is one propeller shaft sealing device that is based on a rotational sealing technique described in the Korea Patent No. 100688250. One embodiment of this propeller shaft sealing device is a metallic tube made of helically coiled metal tapes and is designed to seal rotating bodies using metallic points with rubber-like radial flexibility. This propeller shaft sealing device achieves perfect sealing performance even in deepwater dives and in low temperature waters, such as those in the Arctic Circle.
One drawback of the technology described in the Korea Patent No. 100688250 is that the intended use of implemented sealing device is determined by the rotational sealing direction: clockwise or counter-clockwise. It is because the nature of helical coiling during the manufacturing process, the sealing function is enabled only for a single rotational direction of the sealing device according to the coiling direction. This limits the application of this technology to rotational machineries that do not reverse rotational directions frequently.